The present invention relates to the form of tri-color shadow-mask tube popularly referred to as a black-surround tube. Such a tube is characterized by the fact that the phosphor dots which constitute the phosphor triads of the screen are reduced in size so as to be separated from one another as distinguished from being in tangential contact. A light-absorbing material, such as graphite, is deposited in the spaces which intervene the phosphor dots. In short, the individual phosphor dots are surrounded by a black material, giving rise to the term black-surround screen.
The preferred form of such a tube is described and claimed in U.S. Pat. No. 3,146,368 - Fiore et al, issued Aug. 25, 1964 and assigned to the assignee of the present invention. It has a unique dimensioning of the screen and mask, selected so that the electron beams are larger in cross-sectional area than the phosphor dots. This permits optimizing the improvement in brightness and contrast that inures from the black-surround construction. An alternative form, known commercially as a "matrix" tube, retains the relative beam to dot size of conventional shadow mask tubes in which the beam area is less than the phosphor dots. Where the beams and phosphor dots are of different sizes, with one larger in diameter than the other, there is an overlapping relation and the extent of overlap provides a guard band or tolerance to accommodate some amount of misregistration of the beam and dot without affecting purity. The tolerance is said to be positive where the phosphor dot is larger than the electron beam and is said to be negative where the phosphor dot is smaller. Both described forms of screen structure may exhibit the same guard band or tolerance range but the negative tolerance construction, featuring a larger beam than phosphor dot, is preferred since it accommodates a greater amount of light-absorbing material on the screen from which optimized conditions of contrast and brightness may be realized.
The present invention concerns still a third screen structure which evolves from developments directed to achieving performance results approaching that of the negative tolerance construction but with screening techniques closely resembling those utilized in fabricating the matrix screen structure.
Accordingly, it is an object of the invention to provide a shadow-mask type of tri-color cathode-ray tube having a novel black-surround screen structure.